1. Field of the Invention
The present invention relates to a product provided with antistatic properties.
2. Description of the Background
Appropriate thermoplastics are increasingly replacing metal as a material in piping systems which transport combustible fluids, e.g. motor fuel piping systems for vehicles or aircraft, or supply piping in gas stations. This results in savings in weight and in production costs, but in turn gives rise to the disadvantage that electrostatic charges can arise.
Under certain conditions the electrostatic charge created by the flow of motor fuel or solvent can discharge extremely rapidly and produce holes in the pipeline wall, through which the motor fuel or, respectively, the solvent can escape. On contact with hot components in the vicinity, or if sparks are generated, motor fuel or solvent can ignite and cause a fire in the vehicle or installation. In partly filled tanks where the motor fuel can move around freely, the electrical charge can also cause explosion of ignitable mixtures within these.
It is known that this problem can be avoided by providing the components of the piping system with antistatic properties. For example, DE 40 25 301 C1 describes antistatic motor fuel piping for motor vehicles which is composed of at least two different layers of polymer. At least one of the layers has been modified with electrically conducting additives, such as conductivity black. Although the motor fuel piping is described in that text as peroxide-resistant, it has become apparent that this applies only to layers which do not comprise the carbon black. The test conditions used take no account of the damage to the relatively thin layer provided with antistatic properties.
It has now also been found that the conductivity blacks used hitherto have a disadvantageous catalytic action. Pipes provided with these and having two or more layers show serious deterioration in low-temperature impact strength even after a relatively short time on storage in peroxide-containing motor fuels (sour gas), e.g. to the Ford specification FLTM AZ 105-01, PN180 or the GM specification GM213M, PN50. Another problem which has been found is that the ageing of pipes of this type having two or more layers is markedly more rapid on exposure to heat, e.g. in the engine compartment, with attendant embrittlement.
EP-A-0 730 115 is based upon the recognition that the resistance to peroxide-containing motor fuels of pipes having two or more layers and provided with antistatic properties is improved by using no conductivity black, and using graphite fibrils in its place. However, in practice this improvement is insufficient in many cases. In addition, ageing on exposure to heat continues here to be at a level inappropriately high for practical purposes. A further factor is that graphite fibrils are very expensive.
EP-A-0 745 763 describes a motor fuel filter made from plastic. Its casing is composed of at least three layers, and the inner and the outer layer are composed of a plastic provided with conductive properties. Electrically conducting additives mentioned are, inter alia, conductivity black and graphite fibrils. This motor fuel filter suffers from the disadvantages discussed above.
The object was therefore to produce products provided with antistatic properties and having both high resistance to peroxide-containing motor fuels or solvents and low susceptibility to heat-ageing.
This object was achieved by means of a product made from plastic and composed, at least in part, of a thermoplastic molding composition which comprises polyamide and comprises from 3 to 30% by weight, preferably from 10 to 25% by weight and particularly preferably from 16 to 20% by weight, of a conductivity black, wherein the conductivity black is defined by the following parameters:
a) Dibutyl phthalate (DBP) absorption to ASTM D2414 of from 100 to 300 ml/100 g, preferably from 140 to 270 ml/100 g;
b) a specific surface area, measured via nitrogen absorption to ASTM D3037, of from 30 to 180 m2/g, preferably from 40 to 140 m2/g;
c) an ash content to ASTM D1506 of less than 0.1% by weight, preferably below 0.06% by weight, particularly preferably below 0.04% by weight, and
d) a grit content of not more than 25 ppm, preferably not more than 15 ppm and particularly preferably not more than 10 ppm.
For the purposes of the present invention, grit is hard coke-like particles which arise as a result of cracking reactions in the preparation process.